1. Field of the Invention
The present invention relates to a crystalline glass composition for use as an electrical insulator in a circuit board such as a multilayer circuit board, a sintered crystalline glass obtained by firing the crystalline glass composition, an insulator composition containing a crystalline glass powder comprising the crystalline glass composition and a ceramic powder, an insulating paste prepared by mixing the insulator composition and an organic vehicle and a thick film circuit board having an insulating layer obtained by firing the insulating paste.
More specifically, the present invention relates to an improved method for allowing the crystalline glass composition or the insulating paste to be sintered at a low temperature while endowing the sintered crystalline glass or insulating layer obtained by firing them with a low dielectric constant and a high thermal expansion coefficient.
2. Description of the Related Art
Miniaturization and high integration of various kinds of electronic appliances have been aggressively advanced in recent years as a result of developments in semiconductor elements for integrated circuits such as ICs and LSIs. Consequently, miniaturization and high integration are also required for circuit boards for mounting the semiconductor elements for the integrated circuits, forcing the electrical wiring of the circuit board to be fine and multilayered.
It is known that signals propagating in a conductor are usually more delayed when materials surrounding the conductor have higher dielectric constants. Therefore, the electrical insulator material for high speed propagation is required to have a low dielectric constant.
However, since the thermal expansion coefficient of the low dielectric constant insulator is in general smaller than thermal expansion coefficient of conductors for use, for example, in electrodes, it is a problem that deformation and cracks are generated due to differences in the thermal expansion coefficients when various materials are integrated into multilayers.
In relation to the problem described above, Japanese Unexamined Patent Publication No. 3-4594 proposes to construct an insulating layer having a larger thermal expansion coefficient by using principal components of MgO, SiO.sub.2 and CaO in a specified composition ratio in order to manufacture a capacitor-integration type composite circuit board in which a dielectric layer having a large thermal expansion coefficient is integrated by inserting it between insulating layers, thereby increasing the thermal expansion coefficient of the insulating layer. A principal crystalline phase forsterite (Mg.sub.2 SiO.sub.4) is deposited by calcining at a temperature of 1000.degree. C. to 1300.degree. C. followed by firing at a temperature of 1240.degree. C. to 1340.degree. C. in the insulating layer is disclosed in this publication to increase the thermal expansion coefficient.
However, sintering at a temperature of 1100.degree. C. or less is impossible in the electrical insulators constituting the insulating layers described in the publication above. Therefore, the materials cannot be used for the electrical insulators for the circuit boards in which conductors containing, for example, silver are formed on the surface or inside of them.
Since merwinite (Ca.sub.3 MgSi.sub.2 O.sub.8), monticellite (CaMgSiO.sub.4) and calcium silicate (at least one of CaSiO.sub.3, Ca.sub.3 Si.sub.2 O.sub.7, Ca.sub.2 SiO.sub.4 and Ca.sub.3 SiO.sub.5) are known to be crystalline phases having larger thermal-expansion coefficients than forsterite (Mg.sub.2 SiO.sub.4), the electrical insulators for the insulating layers described in the foregoing publication seem to fall within a composition region where relatively a small amount of these crystalline phases are deposited.